The present invention relates to a hearing aid, and specifically to a hearing aid having an enhanced vent that produces sound having beneficial characteristics.
A custom hearing aid typically includes a vent. The main purpose of the vent is to reduce the “occlusion” effect, which is defined as an unpleasant sensation related to a loud sound of the hearing aid user's voice. The occlusion sound is generated due to vibrations of the ear canal tissues that are generated, e.g., when the wearer speaks.
The intensity of the occlusion sound is drastically increased when a hearing aid user inserts the aid into the ear. This is because the hearing aid blocks the ear canal, thereby forming a closed volume around the ear drum. When this happens, the sound pressure, caused by vibrations of the canal tissues, increases to levels that make it very annoying to the hearing aid users.
In conventional hearing aid designs, a conventional vent forms a passage for the ear from the closed volume near the ear drum to the outside space, thereby allowing a reduction of the occlusion effect. FIG. 3 illustrates the frequency response of a hearing aid with a conventional vent design.
One negative effect of such a vent, however, is to increase the occurrence of an acoustic feedback by letting the amplified sound pressure from the ear canal enter into the microphone, thereby creating a feedback loop. A hearing aid with a vent has a limited stable gain that is determined by the vent cross-section area, vent length, and the distance between the vent opening and the microphone inlet.
The prior art International patent publication WO 92/21218 (“Gauthier”) illustrates a known hearing aid design (see FIG. 1), showing a microphone 10, receiver (speaker) 12, various electrical components 14, 16, 18, and 20 mounted on a printed circuit board 22, a battery housed in a battery compartment 24, and wires 26 running from the printed circuit board 22 to the speaker 12. The speaker has a sound conducting tube 28 that opens to the ear canal of the user.
Gauthier discloses a vent construction that allows an increase in the stable gain of a hearing aid. The hearing aid has a housing 30 that has an air vent passage 32 extending along the length of the housing and conducts sound from the ear canal to outside of the ear. Gauthier's modified vent (referred to as a “tuned passage”) 42 has an opening from the vent 32 into the inside volume 38 of the hearing aid.
However, the vent construction of Gauthier has the following disadvantages:                1. Due to the resonance effects of the Helmholtz resonator that is formed by the opening 42 and the inside volume 38, the frequency response of the hearing aid becomes strongly distorted. In addition to the expected reduction of the gain at low frequencies (due to leaks of sound energy through the vent), the response developed is illustrated in FIG. 4. Also the increased sound pressure inside the shell 38 leaks into the microphone 10 inlet via the gaps in the battery door, creating another feedback path and causing peaks in the response curve near the 1-3 kHz frequency range, as is illustrated in FIG. 4.        2. The wax fumes go through the opening 42 into the inside of the hearing aid 38 and create wax deposits causing corrosion and a consequential malfunction of the electronic parts of the hearing aid.        